This invention relates to a circuit for producing an error signal for use in a negative feedback loop for automatically controlling a quantity capable of being electrically controlled and represented by a frequency of a feedback signal for the loop.
In a conventional negative feedback loop of the type described, it is often the case to feed an a.c. signal of a frequency representative of an actual value of the quantity back to a rectangular pulse generator, such as a monostable multivibrator, for generating a rectangular signal of predetermined pulse width and height to control the duty cycle of the rectangular signal in compliance with the frequency. The rectangular signal is averaged by an integrator to become a variable signal of a variable d.c. voltage, which is compared with a reference signal of a fixed or adjustable reference d.c. voltage indicative of a value predetermined for the quantity. After amplification, an error representative of the difference between the tow d.c. voltages is supplied to means for electrically controlling the quantity to the predetermined value. The automatic control is thus carried out by forming for a change or fluctuation, if any, in the quantity, a negative feedback loop whose closed loop gain is used to compress or nullify the possible change or fluctuation.
It will readily be understood that the reference d.c. voltage, once set at a desired value, has to be strictly stable. The source voltage for the rectangular pulse generator should also be very stable because a fluctuation would otherwise be introduced into the rectangular signal and accordingly into the variable d.c. voltage so as to unavoidably mis-control the quantity. This is also the case with the integrator. The mis-control becomes more serious in a negative feedback loop of the type described, which has a higher closed loop gain. In order to obtain a constant source voltage, use may be made of a Zener diode or a similar constant-voltage diode. The constant voltage obtained thereby, however, is still subject to change from the strict point of view because the obtained voltage depends on the current flowing through the constant voltage diode, the ambient temperature, and others. In addition, the Zener diode is not applicable to a rectangular pulse generator operable at a source voltage of about four volts or lower because Zener diodes have excellent constant voltage and temperature characteristics only at 5 to 6 volts.
Among the quantities mentioned above, the speed, generally expressed in terms of revolutions per minute, of a d.c. motor operable by a dry battery is liable to fluctuate particularly because the terminal voltage of the battery varies in reponse to a change in the motor speed. This invention will therefore be described in the following in specific conjuction with automatic control of the speed of a motor, such as a d.c. or a.c. motor or a rotary engine, although the invention is equally well applicable to automatic control of the output frequency of a voltage-controlled oscillator and other quantities of the nature defined above.